A study was made of the effects of cisplatin, cis-dichlorodiammineplatinum(II) (5-250 mg l-1), on the physiology and fine structure of Tetrahymena. The physiological effects observed were dose-dependent. Endocytosis was inhibited reversibly in all, but late in the high, concentrations. After an initial dose-related increase, due to division of cells most advanced in the cell cycle, proliferation ceased for at least two normal cell generations (6 h) in 50 and 100 mg drug l-1, but for 24 h in 250 mg l-1, after which multiplication was resumed in a dose-dependent manner. Exposure to cisplatin resulted in the appearance of small, refractive granules and platinum (i.e. electron-dense material) accumulated in these granules. Fine structural observations of cells exposed to 250 mg drug l-1 showed nucleolar fusion and appearance initially of lipid droplets, dense granules and autophagosomes. A time-dependent redistribution of cell organelles was revealed by morphometry; in particular, the mitochondria increased in number, but decreased in size. Moreover, after prolonged treatment (24 h) and without cell division, the inner mitochondrial membrane had diminished and the ratio of the inner to the outer mitochondrial membrane was only half of the value for control mitochondria. Concomitantly with this decrease, the cell content of ATP was reduced to a similar extent. The findings indicate a specific action of cisplatin on mitochondria, resembling that induced in Tetrahymena by chloramphenicol and methotrexate.